DESCRIPTION (Adapted from the Applicant's Application): This is a 5-year competitive renewal of a proposal entitled 'Cellular defense response in an insect system.' In the previous proposal the PI has developed an elegant system to investigate the encapsulation of a foreign object by hemocytes from larvae of the moth, Pseudoplusia includens. These studies are significant because encapsulation constitutes a fundamental component of the insect immune response. In this competing renewal, the PI will continue his investigation of the two cell types, plasmatocytes and granular cells that participate in this response. He will also extend previous work on a plasmatocyte spreading peptide (PSP1), which induces plasmatocytes to undergo a change in adhesive state and to release specific effector molecules. The present application has 4 specific aims: 1. To characterize structural features of PSP1 essential for activity; 2, to identify a PSP1 receptor, 3, to identify molecules that mediate plasmatocyte adhesion, and 4, to examine characteristics of plasmatocyte-induced cytotoxicity. The availability of large numbers of hemocytes from Pseudoplusia includens, and the specific techniques and approaches developed in the PI's laboratory are strong arguments in favor of this experimental system, which will provide important comparative insights into immunity functions in arthropod vectors.